1. Field of the Invention
The present invention relates to an electronically-controlled throttle body in which a valve disc is rotatably moved, via a reduction gear mechanism, by a motor driven in response to gas pedal operations.
2. Description of the Related Art
As shown in Japanese Laid-Open Patent Publication No. 2002-266666 (hereinafter referred to as Patent Document 1), an electronically-controlled throttle body has been heretofore known in which an opening degree of a valve disc is controlled, via a reduction gear mechanism, by a motor driven in response to gas pedal operations. In the throttle body shown in this Patent Document 1, in order to increase torque generated by the motor up to the torque needed for controlling an opening degree of the valve disc, a spur gear two-stage reduction gear mechanism is employed that performs two-step speed reduction by transmitting torque generated by the motor from an output gear fixed on the motor shaft to an intermediate gear that is placed parallel to the output gear, and further transmitting the torque transmitted to the intermediate gear to an input gear that is fixed on the valve shaft supporting the valve disc and placed parallel to the intermediate gear.
However, in a conventional electronically-controlled throttle body as described above, since torque is transmitted by the spur gear two-stage reduction gear mechanism, inter-shaft distances each are needed between the output gear shaft and the intermediate gear shaft and between the intermediate gear shaft and the input gear shaft, depending on each gear's pitch radius; therefore, the size of device is determined by those inter-shaft distances, causing a problem in that the device can not be easily downsized.
Moreover, trying to downsize the device needs to limit the pitch radius of each gear, which needs to limit the number of teeth of each gear accordingly; thereby, it becomes difficult to increase reduction gear ratios, resulting in necessary motor driving torque being greater, which has been an inhibiting factor of downsizing the motor.
Moreover, when a spur-gear two-stage reduction gear mechanism is used for the reduction gear mechanism, the output gear fixed on the motor shaft undergoes reaction force from the intermediate gear; the reaction force generates to the motor shaft bending moment whose fulcrum lies on a bearing of the motor shaft; consequently, the motor shaft undergoes both bending moment and driving torque, so that the strength of motor shaft can not be easily decreased. Therefore, the diameter of the motor shaft can not be reduced in order to secure the strength of the motor shaft, which resultantly has been an inhibiting factor of downsizing the motor.
In addition, when the spur-gear two-stage reduction gear mechanism is used for the reduction gear mechanism, the output and input gear shafts are distant from each other, as well as accuracy of inter-shaft distances among three gear shafts, namely, the output gear shaft, the intermediate gear shaft and the input gear shaft that are parallel to one another, needs to be ensured. Therefore, there has been a problem in that, even if the body is tried to be made of resin, for example, intending to cost down, production of the body becomes extremely difficult due to difficulties in ensuring accuracy of inter-shaft distances among those three gear shafts.
Furthermore, in an air-intake pipe of an engine provided with an electronically-controlled throttle body, when outside air temperature is low, moisture condenses into water after the engine stops, and the resultant water freezes when left at an ambient temperature lower than 0° C. In order to prevent condensed water from depositing in the proximity of a throttle valve so as to prevent the throttle valve from being frozen by the water, the throttle body is generally mounted at a relatively high location in the air-intake passage in the engine room. However, when the motor is located higher than a throttle valve shaft in a conventional electronically-controlled throttle body, there is concern about interference with the hood, and in order to avoid the interference, there has been such a problem as the hood line needs to be elevated. Meanwhile, when the motor is located lower than the throttle valve shaft, a hot water heating passage can not be provided on the ground side of the inner wall of the air-intake passage where condensed water is likely to deposit, which has made it difficult to take efficient countermeasures against freezing.